Trunked RF repeater systems have become a mainstay of modern RF communications systems, and are used, for example, by public service organizations (e.g., governmental entities such as counties, fire departments, police departments, etc.). Such RF repeater systems permit a relatively limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). Typical state-of-the-art RF repeater systems are "digitally trunked" and use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trunking" (time-sharing) of the limited number of RF channels among a large number of users.
Briefly, such digitally trunked RF communications systems include a "control" RF channel and multiple "working" RF channels. The working channels are used to carry actual communications traffic (e.g., analog FM, digitized voice, digital data, etc.). The RF control channel is used to carry digital control signals between the repeater sites and user RF transceivers (radio units) in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message.
Assuming a working channel is available, the repeater site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel. This message temporarily assigns the available working channel for use by the requesting transceiver and other callee transceivers specified by the channel request message. The channel assignment message automatically directs the requesting (calling) transceiver and callee transceivers to the available RF working channel for a communications exchange.
When the communication terminates, the transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is thus available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel. An exemplary "single site" trunked RF repeater system is disclosed in the commonly-assigned U.S. Pat. Nos. 4,905,302 and 4,903,321.
Single site trunked RF repeater systems may have an effective coverage area of tens of square miles. It is possible to provide one or more satellite receiving stations (and a single high power transmitting site) if a somewhat larger coverage area is desired. However, some governmental entities and other public service trunking system users may require an RF communications coverage area of hundreds of square miles. In order to provide such very large coverage areas it is necessary to provide multiple RF repeater sites and to automatically coordinate all sites so that a radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other radio transceivers located anywhere in the system coverage area.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site may typically have a central site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
To enable communications from one area to another, however, a switching network referred to herein as a "multisite switch", must be provided to establish control and audio signal pathways between repeaters of different sites. Moreover, such pathways must be set up at the beginning of each call and taken down at the end of each call. For example, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller S1 via an "inbound" digital control message transmitted over the RF control channel that a working or audio channel is requested. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the audio channel assigned to the call. This assigned channel is applicable only within the area covered by the site.
In addition, the site controller sends the channel assignment to the multisite switch 200 which assigns an internal audio slot to the call. The switch 200 also sends a channel request over a control messaging bus to other site controllers having a designated callee within their site area. Audio signals are routed such that audio pathways are created to serve the callee(s) and one or more dispatcher consoles 202 involved in the communication. Upon receiving a channel request, these "secondary" site controllers (in the sense they did not originate the call) assign an RF working channel to the call. Each secondary channel is operative only in the area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch.
Thus, the caller communicates with a unit or group in another area via the multisite switch. The call is initially transmitted to the primary site controller, routed through an assigned audio slot in the switch, and retransmitted by the secondary sites on various assigned channels in those other areas. When the call ends, the primary site controller deactivates the assigned channel for that site and notifies the multisite switch 200 that the call is terminated. The multisite switch 200 propagates an end of call command ("channel drop") to all other site controllers. This releases all working channels assigned to the call and breaks the associated audio rating pathways.
In addition to providing communications between mobile radio units in different areas, the multisite switch 200 provides communications between land-line telephone subscribers and radio units as well as dispatchers and mobile radio units. Land-line telephone subscribers can communicate with radio units by dialing an access number as well as a radio unit (or group) identification number which is routed to the trunked communications system through a central telephone interconnect switch (CTIS) and the multisite switch 200. One or more dispatch consoles 202 is connected to the multisite switch 200 in the same manner as the site controllers 102. Both land-line subscribers and dispatch console operators can issue a channel call request through the multisite switch 200 to a site controller 102 to call for example a mobile radio unit.
Each dispatch console 202 may participate in calls in its area. Thus, when a call comes through the multisite switch 200 from another area to a mobile radio, the switch informs the dispatch console 202 of the call in addition to notifying the corresponding site controller 102. The dispatch operator can then listen or participate in the call. The multisite switch 200 also handles calls to groups of mobile units and/or dispatch consoles by ensuring that the site controllers for all of the callees in the group assign a channel to the group call.
The multisite switch 200 has a distributed architecture. The logical functions and computational workload of the multisite switch 200 are shared by various distributed microprocessor "nodes". Each node is connected either to a site controller 102, dispatch console 202, public and/or private landline telephone exchanges and other components of the overall radio system. Most nodes function as interface modules and include, for example, Master Interface Modules (MIMs) for the nodes coupled to site controllers and Console Interface Modules (CIMs) for the nodes coupled to dispatch consoles. Each node is supported by a switch controller card operated by microprocessors. All of the cards have substantially the same hardware and are interchangeable. Each card acts as a gateway interface into the distributed switch network.
Multisite communication networks are often used by agencies and departments, e.g. local police and fire departments, that require a high level of reliability. Public communication networks like cellular telephone networks can tolerate some degree of unreliability in their communications. First, cellular communications typically do not involve a coordinated, multiple party response to public emergencies. Second, if a communication between two parties is interrupted due to interference, faults in the network software or hardware, etc., the usual consequence is that those two parties are inconvenienced. The communication can be reinstated (perhaps after some delay) over the cellular network, or alternatively, over the landline network.
The consequences of such interruptions for police and fire departments over a radio communication network are significantly more dramatic. In responding to emergencies, these departments must have virtually uninterrupted and immediate communications with multiple parties including individual and groups of radio units in various geographical areas as well as one or more dispatchers. Even minor delays in restoring communications after a multisite switch fault could have disastrous effects. In addition, police and other agencies require and very often use secure private RF communications involving digital encryption and decryption. There is no practical way to quickly reinstate a broken switch communication over an alternate network so that multiple parties can communicate securely over a single communications channel.
The functionality of a multisite communications system vitally depends on the control messaging information reaching all nodes in the network. If a bus wire in the control messaging network is frayed or broken, if there is a poor connection of a node to the bus, or if there is a voltage spike over the network causing faulty operation of the control message bus, the multisite switch will not function properly. Accordingly, there is a critical need in multisite communication systems to provide a very reliable control message network.
The present invention provides a system for enhancing the reliability of a trunked RF multisite communication switch connecting plural site controllers, each site controller coordinating RF communications between multiple radio units in corresponding geographical site areas and the switch. Multiple microprocessor-controlled nodes interface communications between corresponding site controllers to the switch. Each node is connected to the switch back plane with a first supervisory node being located at one end of the back plane and a second end node being located at the opposite end.
Multiple bus lines are included in the back plane. A time division multiplex (TDM) audio bus transfers digital audio information between the nodes. First and second control message buses are provided for transferring control messages between nodes. A bus select line is connected to direct each of the nodes to select one of the first and second message buses. A control line connects the supervisory and end nodes, and the end node monitors the logic state of a signal output on that line by the supervisory node.
The supervisory node polls each node over a currently selected message bus. In particular, the supervisory node identifies the end node connected to the current message bus based on the end node's polling response message. If the end node does not respond to the polling message in a predetermined time period, the supervisory node selects the other message bus and changes the logic state of the signal output on the control line. Upon detecting a change in the logic state of the signal over the control line, the end node also selects the alternate bus. Having both selected the alternate bus, the first and second nodes simultaneously coordinate switching of all nodes on both sides of any control bus break to the alternate bus via the bus select line.